Positive photoresist compositions in use generally comprise an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive substance. For example, photoresist compositions comprising "a combination of a phenolic novolak resin and a naphthoquinonediazide substitution compound" are described in, e.g., U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. Further, an example of the most typical composition comprising "a combination of a cresol-formaldehyde novolak resin and a trihydroxybenzophenone-1,2-naphthoquinonediazidesulfonic acid ester" is described in L. F. Thompson, "Introduction to Microlithography" (ACS Press, No. 2, 19, pp. 112-121).
Many positive photoresists comprising a novolak resin and a photosensitive naphthoquinonediazide compound have been developed and put to practical use so far from the above-described standpoint. These photoresists have produced satisfactory results in the formation of resist patterns having line widths ranging about from 0.8 to 2 .mu.m.
However, the degree of integration in integrated circuits is increasing more and more, and it has become necessary to form an ultrafine pattern having a line width of 0.5 .mu.m or smaller in the production of semiconductor substrates for VLSIs and the like. For attaining the necessary resolving power, the wavelengths of the light sources used for photolithography are decreasing more and more and, as a result, use of far ultraviolet rays and excimer laser beams (XeCl, KrF, ArF, etc.) has come to be investigated.
The prior art resists comprising a novolak and a naphthoquinonediazide compound are unsuitable for use in pattern formation by lithography using far ultraviolet rays or excimer laser beams, because the novolak and the naphthoquinonediazide exhibit intense absorption in the far ultraviolet region to render the light less apt to reach the resist bottom. Thus, the resist has low sensitivity to give only a tapered pattern.
One means for eliminating the above problem is the chemically amplified resist composition described in, e.g., U.S. Pat. No. 4,491,628 and European Patent 249,139. A chemically amplified positive resist composition is a pattern-forming material in which an acid generates in exposed areas upon irradiation with a radiation such as far ultraviolet rays and this acid catalyzes a reaction that makes the areas irradiated with the actinic rays and the unirradiated areas which are different in solubility in a developing solution to thereby form a pattern on a substrate.
Examples thereof include combinations of a compound which generates an acid upon photodecomposition with an acetal or O,N-acetal compound (see JP-A-48-89003; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), with an orthoester or amidoacetal compound (see JP-A-51-120714), with a polymer having acetal or ketal groups in the backbone (see JP-A-53-133429), with an enol ether compound (see JP-A-55-12995), with an N-acyliminocarbonic acid compound (see JP--A-55-126236), with a polymer having orthoester groups in the backbone (see JP-A-56-17345), with a tertiary alkyl ester compound (see JP-A-60-3625), with a silyl ester compound (see JP-A-60-10247), and with a silyl ether compound (see JP-A-60-37549 and JP-A-60-121446). These combinations exhibit high photosensitivity since they have a quantum efficiency exceeding 1 in principle.
Another means for eliminating the problem described hereinabove is a system which is stable over long at room temperature but decomposes upon heating in the presence of an acid to become alkali-soluble. Examples thereof include systems comprising a combination of a compound which generates an acid upon exposure to light with an ester having a tertiary or secondary carbon (e.g., t-butyl or 2-cyclohexenyl) or with a carbonic ester compound, as described in, e.g., JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642; Polym. Eng. Sce., Vol. 23, p. 1012 (1983); ACS. Sym., Vol. 242, p. 11 (1984); Semiconductor World, p. 91 (November 1987); Macromolecules, Vol. 21, p. 1475 (1988); and SPIE, Vol. 920, p. 42 (1988). Since these systems also have high sensitivity and exhibit reduced absorption in the deep UV region as compared with the naphthoquinonediazide/novolak resin systems, they can be effective systems for coping with the wavelength reduction in light sources.
The chemically amplified positive resists described above are roughly divided into two groups: three-component systems comprising an alkali-soluble resin, a compound which generates an acid upon exposure to a radiation (photo-acid generator), and a dissolution inhibitive compound for the alkali-soluble resin which has acid-decomposable groups; and two-component systems comprising a resin which decomposes upon reaction with an acid to become alkali-soluble and a photo-acid generator.
In these two-component or three-component, chemically amplified positive resists, the photo-acid generator is caused to generate an acid by exposure to light and the resists are then heat-treated and developed in the presence of the acid to obtain a resist pattern.
Known photo-acid generators for use in the above-described chemically amplified positive resists include N-imidosulfonates, N-oximesulfonates, o-nitrobenzylsulfonates, and pyrogallol trismethanesulfonate. Typical compounds which have been used as photo-acid generators having a high photodecomposition efficiency and excellent image-forming properties are the sulfonium and iodonium salts of perfluorinated Lewis acids, e.g., PF.sub.6.sup.-, AsF.sub.6.sup.-, and SbF.sub.6.sup.-, described in, e.g., JP-A-59-45439 and Polym. Eng. Sci., 23, 1012 (1983).
However, these prior art photo-acid generators, when used in resist materials for semiconductors, have a problem that the counter anions of the photo-acid generators cause pollution by phosphorus, arsenic, antimony, etc.
Used as a sulfonium or iodonium compound free from the pollution is the salt described in, e.g., JP-A-63-27829, JP-A-2-25850, JP-A-2-150848, JP-A-5-134414, and JP-A-5-232705, in which the counter anion is a trifluoromethanesulfonate anion.
It should, however, be noted that this prior art composition has a problem that since trifluoromethanesulfonic acid, which generates upon exposure to light, diffuses relatively rapidly in the resist film, the line width of the resist pattern which is being produced becomes narrower with the lapse of time from exposure to light to heat treatment or the resist pattern comes to have a T-top surface.
Although use of a toluenesulfonate anion as another counter anion for sulfonium or iodonium is described in, e.g., JP-A-2-25850, JP-A-2-150848, JP-A-6-43653, and JP-A-6-123972, this salt has a problem that since it has insufficient solubility in ordinary resist solvents, the addition amount thereof is limited, resulting in insufficient sensitivity.
Furthermore, JP-A-6-199770 describes the use of an arylbenzenesulfonate anion having a linear hydrocarbon substituent as a still another counter anion for sulfonium and iodonium salts. The sulfonium and iodonium salts containing the arylbenzenesulfonate anion have improved solubility in ordinary resist solvents. However, this prior art technique also has the problem that the resist pattern which is being formed comes to have a reduced line width or a T-top surface with the lapse of time from exposure to heat treatment.
It has become apparent that the above change in resist pattern with the lapse of time from exposure to heat treatment varies considerably depending on the kinds of the groups (acid-decomposable groups) contributing to image formation, which decompose by the action of an acid.
For example, the aforementioned two-component resists employing tertiary alkyl ester groups as acid-decomposable groups tend to give a resist pattern which comes to have a T-top surface with the lapse of time from exposure to heat treatment, and it has been difficult to diminish the formation of a T-top surface. This is an intrinsic problem of resists employing acid-decomposable groups which decompose at a low rate by the action of an acid, such as tertiary alkyl ester groups. Specifically, in a resist pattern containing such slowly acid-decomposable groups, there is a large difference between the amount of the acid-decomposable groups decomposed by an acid generated just after exposure and the amount of the acid-decomposable groups decomposed by heat treatment after the exposure. As a result, the resist surface, which is more influenced by contamination with amines contained in the atmosphere and by the diffusion of the acid into the air, is apt to undergo a change in profile, i.e., the formation of a T-top surface, with the lapse of time from exposure to heat treatment. Therefore, it is virtually impossible in single-layer resists to use not-readily acid-decomposable groups, such as tertiary alkyl ester groups, as the only acid-decomposable groups for the purpose of image formation, though there is an exception that the above problem is overcome by regulating the glass transition point of a binder to thereby lower the glass transition point of the whole resist film and by elevating the baking temperature in film formation.
On the other hand, resists containing a silyl ether compound or an acetal compound frequently have the problem that the resist pattern comes to have a reduced line width with the lapse of time from exposure to heat treatment. This phenomenon is thought to occur by the following mechanism. In a resist containing a compound which is acid-decomposable at a high rate, such as a silyl ether or acetal compound, the decomposition of the compound by the acid which has generated just after exposure proceeds to a degree sufficient for image formation. With the lapse of time from exposure to heat treatment, the acid-decomposable compound decomposes due to the horizontal diffusion of the acid and further decomposes gradually at room temperature by the action of the acid remaining in a slight amount in half-exposed areas. As a result, the resist pattern comes to have a reduced line width. It has however been found that the above phenomenon proceeds to different degrees depending on the kinds of photo-acid generators.
However, especially the resists in which such readily acid-decomposable groups are utilized for image formation have been found to have the following new problem besides the problem that the resist pattern comes to have a reduced line width with the lapse of time from exposure to heat treatment. That is, the resist pattern comes to have a deteriorated profile due to residual standing wave, and undergoes collapse as a result of undercut at the interface with the substrate. This new problem has been found to be severer especially in resists containing a photo-acid generator of the above-described sulfonium or iodonium salt type.
The profile deterioration described above is a serious problem which should be overcome in resists containing a sulfonium or iodonium salt type photo-acid generator. This is because these resists specifically have higher sensitivity and higher resolving power than the resists containing any of the other known photo-acid generators, although the reason for this has not been elucidated.
On the other hand, JP-A-5-181279, JP-A-5-323590, and JP-A-6-130666 disclose that the resist profile deterioration which occurs with the lapse of time mainly from exposure to heat treatment can be prevented by using two kinds of photo-acid generators in combination.
For example, in JP-A-5-181279, there is a description to the effect that a resist surface can be inhibited from becoming soluble, i.e., coming to form a T-top surface, with the lapse of time from exposure to heat treatment by using a combination of a compound (photo-acid generator) which generates a strong acid (e.g., a sulfonic acid) upon exposure and a compound which generates a weak acid (e.g., a carboxylic acid) upon exposure.
In JP-A-5-323590 is described a technique for changing the solubility of a resist film in a developing solution after exposure. In this technique, a compound contributing to image formation (photo-acid generator) and a compound not contributing to image formation are used in combination to increase the absolute amount of the acid generated after exposure and thereby reduce the relative amount of the acid consumed in a resist surface layer by impurities present in the atmosphere. Thus, according to the above reference, the resist surface can be inhibited from becoming soluble, i.e., coming to form a T-top surface, with the lapse of time from exposure to heat treatment to thereby obtain heightened resolving power.
In JP-A-6-130666, there is a description to the same effect that use of a combination of a strong acid and a weak acid is effective in inhibiting a resist film from undergoing a change in performance with the lapse of time from exposure to heat treatment.
However, any combination of compounds according to the prior art techniques is insufficient in resolution and in inhibiting a resist pattern from coming to have a reduced line width with the lapse of time from exposure to heat treatment. In addition, those prior art techniques have failed to give satisfactory results from the standpoint of preventing collapse.
As described above, in the prior art techniques, it has not been fully known how to design a positive photosensitive composition which satisfies all the desired properties, i.e., high sensitivity, high resolving power, and freedom from a decrease in resist pattern line width or from the formation of a T-top resist pattern surface with the lapse of time from exposure to heat treatment, and, especially in a resist system utilizing readily acid-decomposable groups for image formation, less profile deterioration such as residual standing wave and collapse.